A multi-tone, frequency hopped RF-lightwave waveform functions as a lightwave carrier for an optical transmission channel. The RF signal information carried by the optical transmission channel may be a pulse code, for example, which may be imposed onto the frequency-spread RF-lightwave carrier by means of a lightwave modulator. The final RF-lightwave waveform can be transmitted (by means of an optical fiber link or a free-space optical link) to a photoreceiver. The photoreceived signal, which is in electronic form (frequency converted and demodulated), can then be transmitted through a RF channel (an antenna or wireless link).
As is disclosed herein, the generator of the RF-lightwave carrier includes a frequency-comb generator that is coupled to an optical-heterodyne synthesizer. The comb is a set of RF tones amplitude-modulated onto a lightwave carrier. The generator of the RF-lightwave frequency comb is preferably a photonic oscillator, whose construction is known in the art. The optical heterodyne synthesizer is switchable and produces a pair of phase-locked, CW lightwave lines (at two different optical wavelengths). One of these lightwave lines has the RF comb modulated onto it. Both lines, after being modulated by the comb, are then combined to generate the agile carrier. The center frequency of the photoreceived signal is the heterodyne beat note, which is the difference between the frequencies of the two lightwave lines produced by the optical heterodyne synthesizer. The wavelengths of these lines can be changed rapidly (the wavelengths of these lines can be changed with each transmit pulse, within a single transmit pulse or even within the transmission of a packet of data) to produce different beat-note frequencies. This process hops the center frequency of the resultant multi-tone RF lightwave carrier. Various known methods can be used to realize the optical heterodyne synthesizer.
One purpose of the agile frequency spreading and hopping is to make the resultant signal difficult for a non-coherent receiver to detect. Use of a frequency-spread carrier is one method to produce a signal that has Low Probability of Interception (LPI) by conventional intercept receivers. In addition, if the precise frequency of the carrier can be changed and is unknown to the interceptor, LPI performance is enhanced. These techniques are useful in LPI radar and communication systems.
Typically, an interceptor would use a wideband receiver that is channelized into smaller frequency bands to detect and identify the signal. If the signal falls within a single channel of the receiver, then it can be detected. However, if the signal is spread in frequency so those portions of it fall within many channels, it is difficult for the interceptor to distinguish that signal from the background noise. Typically, the channels of the intercept receiver may be scanned or long integration times may be used to sense an incoming signal. If the signal frequency is varied rapidly to hop between different channels within the sensing time, it again appears like noise. Alternatively, if the signal frequencies are varied rapidly with time although those hops lie within the received channels, that signal will be detected but difficult to identify.
Another purpose of the frequency spreading is to make the signal less susceptible to jamming. The frequency coverage of the jammer may not be as large as the coverage of the frequency-spread carrier. In addition, since the frequency-spread carrier consists of discrete tones that are can be summed coherently, the signal power is used more efficiently. This is in contrast to the jammer, which is uniformly broadband. Rapid switching of the signal band also makes it less susceptible to being jammed, since the jammer cannot predict from one signal pulse to the next which frequency to jam.
Previous methods to achieve LPI performance are based on using electronic synthesizers to produce the waveforms. Typically, a pulse-compression code is used to phase modulate a single-tone carrier and spread the spectrum. For example, if the signal pulse is 1 μsec wide and a 100-to-1 pulse compression code is used, a signal bandwidth of 100 MHz is obtained. The channel bandwidth of the interrogating receiver is typically much narrower than this. The bandwidth of present high-dynamic-range analog-to-digital converters is typically 100 MHz or less. Thus, interrogator channel bandwidths are also 100 MHz or less. This invention preferably makes use of the wideband nature of photonics to generate the frequency-spread waveforms. The total bandwidth of the comb can be quite wide, with several tens of GHz bandwidths easily achieved by the photonic methods of this invention. A pulse-compression code may be modulated onto the multi-tone comb, in addition to the signal information, to further spread the carrier. Prior art digital synthesizers which produce frequency-stepped waveforms typically have a bandwidth of less than 100 MHz. The switchable, optical-heterodyne synthesizer disclosed herein is capable of a frequency range that exceeds 100 GHz.
The agile frequency spread waveform generator disclosed herein also is useful for communication systems with multiple users. Each user is assigned a particular and unique pattern for the frequency hops of the multi-tone waveform. A user can distinguish its signal from other signals that occupy the same band of frequencies by coherently processing the received signal with a copy of the particular waveform pattern of that user. This type of Code Division Multiple Access (CDMA) for lightwave waveforms is different from prior methods. The prior methods make use of short optical pulses, much shorter than the information pulse, whose wavelength and temporal location can be different for each user.
The Prior Art Includes:
    1. A single-tone, single-loop optoelectronic oscillator—see U.S. Pat. No. 5,723,856 issued Mar. 3, 1998 and the article by S. Yao and L. Maleki, IEEE J. Quantum Electronics, v.32, n.7, pp.1141-1149, 1996. A photonic oscillator is disclosed (called an optoelectronic oscillator by the authors). This oscillator includes a single laser and a closed loop comprised of a modulator, a length of optical fiber, and photodetector, an RF amplifier and an electronic filter. The closed loop of this oscillator bears some similarity to the present invention. However, the intent of this prior art technique is to generate a single tone by incorporating an electronic narrow-band frequency filter in the loop. A tone that has low phase noise is achieved by using a long length of the aforementioned fiber. Demonstration of multiple tones is reported in this article achieved by enlarging the bandwidth of the filter. However, the frequency spacing of those multiple tones was set by injecting a sinusoidal electrical signal into the modulator. The frequency of the injected signal is equal to the spacing of the tones. This method causes all of the oscillator modes (one tone per mode) to oscillate in phase. As a result, the output of this prior art oscillator is a series of pulses. See FIG. 14(b) of this article.    2. A single-tone, multiple-loop optoelectronic oscillator—see U.S. Pat. No. 5,777,778 issued Jul. 7, 1998 and the article by S. Yao and L. Maleki, IEEE J. Quantum Electronics, v.36, n.1, pp.79-84, 2000. An optoelectronic oscillator is disclosed that uses multiple optical fiber loops, as the time-delay paths. One fiber loop has a long length and serves as a storage medium to increase the Q of the oscillator. The other the fiber loop has a very short length, typically 0.2 to 2 m, and acts to separate the tones enough so that a RF filter can be inserted in the loop to select a single tone. The lengths of the two loops, as well as the pass band of the RF filter, can be changed to tune the frequency of the single tone that is generated. This approach teaches away from the use of multiple optical loops to obtain multiple tones, since it uses the second loop to ensure that only a single tone is produced.    3. 1.8-THz bandwidth, tunable RF-comb generator with optical-wavelength reference—see the article by S. Bennett et al. Photonics Technol. Letters, Vol. 11, No. 5, pp. 551-553, 1999. This article describes multi-tone RF-lightwave comb generation using the concept of successive phase modulation of a laser lightwave carrier in an amplified re-circulating fiber loop. The lightwave carrier is supplied by a single input laser whose optical CW waveform is injected into a closed fiber loop that includes an optical phase modulator driven by an external RF generator. This results in an optical comb that has a frequency spacing determined by the RF frequency applied to the phase modulator and absolute frequencies determined by the wavelength of the input laser. The loop also contains an Er-doped optical fiber amplifier segment that is pumped by a separate pump laser. The effect of the optical amplifier in the re-circulating loop is to enhance the number of comb lines at the output of the comb generator. One may expect some mutual phase locking between the different comb lines since they are defined by the phase modulation imposed by the external RF generator.    4. One technique for generating a RF signal is by optical heterodyning. See FIG. 1. With this technique, the optical outputs of two laser wavelengths produced by a RF-lightwave synthesizer are combined onto a photodetector. In one simple case, the RF-lightwave synthesizer consists of two lasers each producing single wavelengths, i.e., single spectral lines. When their combined output is converted by a photodetector into an electronic signal (the photocurrent), that electronic signal has frequency components at the sum and difference of the two laser lines. Typically, the photodetector also acts as a low-pass frequency filter so that only the heterodyne difference frequency is produced. In order for the heterodyne output to be produced, the two laser lines must be locked together, so that their fluctuations are coherent. Various methods known in the art can be employed to achieve this locking. Optical heterodyning also can be combined with an external optical modulator to perform frequency conversion (frequency translation). This function is illustrated in FIG. 1. The dual-line lightwave output of the RF-lightwave synthesizer is supplied to an optical intensity modulator, with a typical modulator being a Mach-Zehnder interferometer. A RF input signal is also supplied to the modulator, which applies an intensity modulation onto the lightwave signal. The transfer function of the modulator results in the generation of frequency sum and difference terms. The output of the photodetector is another RF signal with frequency components that are the sum and difference between the frequencies of the RF input ωR F and the frequency spacing between the two laser lines. In essence, the frequency difference ωLO of the two laser-lines acts as a local-oscillator (LO) frequency that is multiplied with the RF input signal to produce an intermediate frequency (IF) ωLO−ωRF. A mathematical expression for this process is given as:
      i    D    =                    α        ⁢                                  ⁢                  I          o                            2        ⁢                  L          MOD                      ⁢          {              1        +                  m          ⁢                                          ⁢                      sin            ⁡                          (                                                ω                  RF                                ⁢                t                            )                                      +                              M            ⁢                                                  ⁢                          cos              ⁡                              (                                                                            ω                      LO                                        ⁢                    t                                    +                  ϕ                                )                                              ±                                    1              2                        ⁢            mM            ⁢                                                  ⁢                          sin              ⁡                              [                                                                            (                                                                        ω                          LO                                                ±                                                  ω                          RF                                                                    )                                        ⁢                    t                                    +                  ϕ                                ]                                                        }      where iD is the photocurrent.